The present invention relates to a position control apparatus for a moving body, and in particular to a moving body position control apparatus suitable to control a device having operation characteristics largely affected by position variation, such as a high-precision servomotor and a video tape recorder driven by a motor of this kind.
It is desired that the velocity of a drive motor for a VTR (video tape recorder) is constant. Velocity variation (such as rotation unevenness, velocity ripple, or torque ripple) disturbs images, resulting in significantly impaired reliability and quality of the VTR.
In apparatuses of this kind, DC motors were mainly used. In recent years, brushless motors each running at a speed which can be changed freely and easily are adopted in increasing examples. Since the brushless motors have no mechanical brushes, various problems caused by abrasion or abrasion powder of brushes or commutators are removed. On the other hand, brushless motors of a 120-degree conduction system have a drawback that torque ripple and hence rotation unevenness (velocity variation) at the time of operation are caused by flux linkage of the conducting coil differing depending upon the position of the rotor.
In general, VTR motors are controlled by position control and velocity control. As a method for mitigating the above described drawback, therefore, there has been proposed in JP-A-1-308188 (U.S. Pat. No. 4,914,361) a scheme in which the gain of the velocity control system is changed according to the operation state of the motor. However, this scheme has a drawback that selection of control constants becomes complicated because of the mixed presence of both position and velocity control systems and rotation unevenness becomes large depending upon the selection.
As another method for mitigating the above described drawback, a scheme in which position control, i.e., a PLL (phase locked loop), is applied to a motor for VTR (especially a cylinder motor) is conceivable.
Since rotation unevenness changes according to the gain of the position control system (PLL control system), reduction of rotation unevenness can be achieved by increasing the gain of the position control system. If the gain of the position control system is excessively increased, however, the system becomes unstable and rotation unevenness is aggravated in some cases. An optimum gain for reducing the rotation unevenness exists in the control system. The optimum value of the gain of the position control system influencing this rotation unevenness is affected by the torque constant of the motor, resistance and so on. Since the optimum value of the gain depends upon the temperature, dispersion in resistance values of resistors in use, and so on, it assumes different values in individual control systems. In this technique of the prior art, therefore, the gain is set under the worst condition with due regard to quality dispersion in final products, for example.
On the other hand, a feedback control method of changing the feedback gain on the basis of a deviation value which is the difference between an angle actually detected by an encoder and a command value, is described in JP-A-63-287377.
Further, there is described in JP-A-63-15303 a learning control method, whereby in a playback robot the gain the next time is set on the basis of the error of the last time whenever the whole positioning process is finished.
In the control of the first conventional technique described above, such a value of control gain that the system may not become unstable even under the worst condition is selected in conformity with the worst condition. In a control apparatus operated at usual temperatures or in a control apparatus adjusted with no resistance dispersion or variations, therefore, the apparatus cannot be operated with the minimum rotation unevenness, resulting in a problem.
In the succeeding description of the feedback control method, the gain is changed according to the value of deviation. However, changing the gain according to the direction of temporal change of the deviation value is not described. That is to say, unless the gain is increased or decreased according to the direction of deviation value even if the deviation value is the same, the control system might become unstable.
Further, in the description of the aforementioned learning control scheme, the next positioning control gain is set whenever the positioning step is finished. During the positioning operation, however, the operation is conducted without changing the gain which has been set the last time. It is thus impossible to cope with a change of load or input power caused during the positioning operation for some reason, resulting in a problem.